In holographic storage, data is stored in a hologram resulting from the interference of a signal and a reference beam. During storage, both the reference and the signal beams are incident on the storage medium. During retrieval, only the reference beam is incident on the medium. The reference beam interacts with the stored hologram, generating a reconstructed signal beam proportional to the original signal beam used to store the hologram. Relative to conventional magnetic and optical data storage methods, holographic data storage promises high storage densities, short access times, and fast data transfer rates. The widespread use of holographic data storage has been hindered in part by the relative complexity of the specialized components required for storage and retrieval of data.
For information on conventional volume holographic storage see for example U.S. Pat. Nos. 4,920,220, 5,450,218, and 5,440,669. In conventional volume holographic storage, each bit is stored as a hologram extending over the entire volume of the storage medium. Multiple bits are encoded and decoded together in pages, or two-dimensional arrays of bits. Multiple pages are stored within the volume by angular, wavelength, phase-code, or related multiplexing techniques. Each page can be independently retrieved using its corresponding reference beam. The parallel nature of the storage approach allows high transfer rates and short access times, since as many as 106 bits within one page can be stored and retrieved simultaneously.
In a conventional angular multiplexing scheme, the angle between the signal beam and the reference beam is changed. Such a process is normally achieved by a combination of an angularly tunable mirror and an optical relay system, as is shown in FIG. 1. A reference beam 112 is reflected by an angularly tunable mirror 103, such as a galvanometer, in a first position 100. The light spot 102 on the angularly tunable mirror is imaged in the center of a holographic storage crystal 104 by a 4F relay imaging system. The reflected beam 116 passes through two lenses 106 and 108, which have the same focal length F, and interferes with a signal beam 114 in the holographic storage medium 104. The angularly tunable mirror 103 is placed at the focal plane of the lens 106. The distance between the two lenses 106 and 108 is 2F. The center plane 110 between the two lenses 106 and 108 is the Fourier plane of the lens 106. The holographic storage medium 104 is positioned at a distance of F from the lens 108. When the angularly tunable mirror 103 rotates to a second position 101, a second reflected reference beam 118 passes through lenses 106 and 108, enters the holographic storage medium 104, and interferes with a signal beam 114 at the same position and yet a different angle with respect to the beam 116. The relay performance of a conventional refractive optical system is inversely proportional to the range of angles it is designed to relay. This angular multiplexing system is usually space demanding. Furthermore, since the lenses 106 and 108 are not monolithic, an optical alignment procedure is required before use.
U.S. Pat. No. 5,671,073 taught a shift multiplexing method. A spherical wave or a fan of plane waves can be used as the reference which interacts with a signal beam in a holographic storage medium at an angle. In fact, different parts of the reference interact with the signal beam at slightly different angles. The holographic storage medium is shifted at predetermined distances with respect to the signal and reference beams in order to record different pages of data. Different parts of the reference contribute to the writing and reading of different holograms at different displacements. Shift multiplexing can be considered as another form of angular multiplexing.
An imaging system using all reflective optics has been disclosed in the U.S. Pat. No. 3,190,171. The prior art teaches the construction of a viewing device using a relay imaging system. This relay imaging system uses concave and convex mirrors. Similar systems have also been taught in U.S. Pat. Nos. 4,796,984, and 4,293,186. The concave-convex-mirror imaging system has excellent off-axis optical performance. Application of this system to lithography technology has been taught in U.S. Pat. No. 3,748,015, and in A. Offner's article: "New Concepts in Projection Mask Aligners", OPTICAL ENGINEERING, Vol. 14, No. 2, 1975.